The present invention relates to mixer systems, and particularly to systems (methods and apparatus) for the circulation and gas-liquid contacting of liquids in a tank, especially when such liquids have non-Newtonian, shear thinning viscosity characteristics. Good circulation and mixing of the liquid and intimate gas-liquid contacting facilitates mass transfer of a gaseous component into the liquid.
The invention is especially suitable for use in bio-reaction processes, such as fermentation by circulating slurries containing microbes and growth media, especially where the fermentation process increases the viscosity of the slurry. The present invention enables improved oxygenation and mixing of such liquids to promote the fermentation process. A fermentation process in which the invention finds particular application is a process for producing polysaccharides such as xanthan gum, and improves such process by enabling increased circulation and mixing of the solution and oxygenation thereof at high concentrations of xanthan gum which results in such high viscosities so as to preclude effective circulation and mixing thereof by conventional means, such that the value of the product of the fermentation, which is a function of the concentration of xanthan gum, is increased or produced in a shorter period of time.
Non-Newtonian liquids which can be effectively mixed and oxygenated with a mixing system embodying the invention have shear thinning characteristics, that is the viscosity of such liquids decreases significantly in the presence of shear. Regions in which shear is produced, in the attempt to reduce the viscosity of the liquid so as to enable it to be circulated, have in conventional systems been confined to the immediate vicinity of the impellers used to circulate the liquid. Such regions have sometimes been referred to as caverns of shear thinned liquid surrounding the impellers. The remaining liquid, for example in a tank in which the impellers are located, remains at high viscosity and thus does not circulate or mix to the extent required for effective gas transfer, and particularly oxygenation of the entire body of liquid in the tank. These non-mixed or non-circulating portions of the tank liquid are often referred to as xe2x80x9cdead zonesxe2x80x9d and significantly reduce the overall effectiveness of the fermentation process.
It has been discovered in accordance with the invention that circulation of a substantial volume fraction of the liquid in the tank enables circulation of the entire body of liquid in the tank. In the case of shear thinning (non-Newtonian) liquids, a shear field or pattern of agitation effects circulation of the liquid when it is achieved within a substantial volume of the liquid in a tank. Shear fields or patterns which produce flow in a direction other than the direction of circulation, for example, a swirling flow, is inhibited in accordance with the invention. The flow through the substantial volume of liquid causes flow elsewhere throughout the tank thereby circulating the entire body of liquid to obtain good top to bottom turnover of the liquid in the tank. The introduction of gas into the circulating flow and the gasification thereof as may be required by the process, for example a fermentation process ongoing in the tank, is then achievable.
It has been proposed to use various expedients for enhancing mixing and circulation in a tank. However, these techniques have been unable to provide adequate circulation and mixing at flow rates sufficient to facilitate the process under the severe circulation and mixing conditions such as presented by many non-Newtonian, shear thinning liquids, especially in fermentation processes.
Accordingly it is an object of the invention to provide an improved mixing system which enables effective mixing and circulation of liquids under severe mixing conditions, especially those presented by non-Newtonian (shear thinning) liquids.
It is a still further object of the present invention to provide an improved system involving circulation of liquids and the gasification thereof which can be carried out effectively with high viscosity, shear thinning liquids.
It is a still further object of the present invention to provide improved impeller systems, which effect circulation of liquids in a tank, which are efficient in terms of the power required to produce a required flow in the tank.
It is a still further object of the present invention to provide an improved impeller which facilitates surface gasification by creating a spray of liquid above the surface of the liquid in the tank, such surface gasification being referred to herein as surface aeration, without limitation to the nature of the gas (whether air or oxygen or some other gas) at the surface of the liquid in the tank.
It is still a further object of the invention to provide a mixing environment that reduces the apparent viscosity of the solution and thereby increases the liquid phase mass transfer and thus increases the overall gas-liquid mass transfer.
It is a still further object of the present invention to provide an improved method of determining the efficiency of gasification which is referred to herein as mass transfer of the gas to the liquid in terms of an overall liquid phase mass transfer coefficient, kLa, and particularly to a method for measurement of the oxygenation of the liquid by unsteady state reaeration so as to enable such measurements to be accurately made where standard dissolved oxygen probes and standard Winkler dissolved oxygen titration procedures are not useful because of the high viscosity and ineffective mixing of the bulk liquid phase and the opaqueness of the aerated liquid medium.
Briefly described, the present invention may be embodied in a mixer system disposed below the surface of the liquid in a tank (the surface being measured when the liquid is static, as when not being circulated) and utilizes a plurality of impellers spaced from each other along the axis of a stationary draft tube, around which axis the impellers are rotated. The draft tube provides coaxial regions inside and outside of the tube, with the diameter of the tube and its length being such that the tube occupies a substantial volume fraction of the liquid in the tank. The impellers include a plurality of impellers, and the impellers create a shear field or pattern of agitation and a pressure gradient to produce good circulation upwardly through the inside region and then downwardly through the outside region. The impellers provide agitation fields which are coupled to each other, and particularly which overlap. Swirling flow inside the draft tube is inhibited, for example, by baffles which project radially inwardly from the draft tube wall and axially between the impellers and preferably above and below the upper and lower most impellers.
Gas may be sparged (injected) into the flow entering the draft tube and/or at the liquid surface. In such event the gasification by entrainment of gas in the tank above the liquid surface may be enhanced by the use of a surface aeration impeller. Also the circulation at the top of the draft tube may be enhanced by a shroud which bridges the inner and outer regions.
The surface aeration impeller may be provided by a plurality of blades spaced circumferentially from each other and disposed at acute angles to radial lines from the axis of rotation of the impeller. The lower portions of the blades, which may extend below the surface, may be folded outwardly. The blades drive the liquid into a spray umbrella in a direction upwardly and outwardly away from the surface.